1. Field of the Invention
The present invention relates to an arrangement for determining an optimal opening pressure in a lung system, of the type having a regulating unit for supplying a respiratory gas to the lung system, a measuring unit for measuring at least one respiratory gas parameter and a control unit for determining the opening pressure based on the measured parameter(s).
The present invention also relates to a method for determining an optimal opening pressure in a lung system.
2. Description of the Prior Art
In a healthy lung, air passes during inspiration through the airways and bronchi to the alveoli in the lungs. An exchange of gas occurs in the alveoli, whereby blood is oxygenated and carbon dioxide is simultaneously transferred to air in the alveoli. Gas containing carbon dioxide is extracted from the alveoli during expiration, permitting the entry of fresh air during the following inspiration. Since a healthy lung has a large compliance, i.e. a high flexibility, a relatively large volume of air can be inhaled in each breath without excessive increases in the pressure of air in the lungs.
As a result of injuries or disease, the function of the lung can be effected to such a degree that a life threatening condition could develop. For example the alveoli might collapse, thereby impairing or, worse, preventing the essential exchange of gas between air in the alveoli and blood in the minute capillaries of the lungs. The lungs can also have atelectatic regions which reduce the compliance so much that an insufficient volume of air is inhaled in each breath. Connection of the damaged lung to a ventilator/respirator may then be necessary to keep a patient alive until the damaged lung has healed. A ventilator/respirator can deliver a respiratory gas to the lungs with a pressure high enough to open the collapsed alveoli in order to provide a sufficient gas exchange. The necessity of a high pressure follows the Laplace Law, P=2.gamma./r, where P is pressure, .gamma. is surface tension and r is radius. A collapsed alveolus has a very small radius, whereas an open alveoli has a (relatively) large radius, thereby requiring a lower pressure to remain open or to be further inflated. In a healthy lung, the alveoli have a layer of natural surfactant. The natural surfactant has the ability of varying its surface tension, thereby for the healthy lung keeping even the minute alveoli open at fairly low pressures. In a pathological condition, such as ARDS (Acute Respiratory Distress Syndrome), however, the alveoli may be depleted of surfactant, resulting in a constant, high air-tissue surface tension in the alveoli. This, of course, makes it even more difficult to open the collapsed alveoli.
The importance of opening the lungs and keeping them open is further described in an article entitled "Open up the lung and keep the lung open" by B. Lachmann, Intensive Care Medicine (1992) 18:319-321. Air at a relatively high pressure must be supplied to the lungs in order to force the alveoli to open, whereas a much lower pressure is required to keep the alveoli open, once they have been opened properly. At the same time, the risk of lung trauma in forced respiration increases at higher pressures (barotrauma) and/or larger volumes of respiratory gas (volume trauma), especially if lung compliance is simultaneously poor. Another risk connected with excessive pressures is that the capillaries can be damaged by shear forces developing within the lung or can be compressed so that the blood cannot flow through the capillaries (over-distension), thereby also preventing a gas exchange. In the aforementioned article it is also noted that a sufficient partial pressure of oxygen P.sub.a O.sub.2 in the blood is a sign of the efficiency of gas exchange in the alveoli, i.e., a measure of the degree to which the lungs are open.
In published Swedish Application 501 560 a ventilator/respirator is described which can determine an opening pressure of a lung based on the relation between measured pressure values and volume values of a respiratory gas supplied to a patient. Basically the ventilator delivers a constant flow of respiratory gas during inspiration and the resulting calculated. As long as the alveoli are collapsed the pressure will increase rapidly whereas pressure in the lung is measured the volume only increases slowly. As the alveoli open, the volume entering the lungs will increase more rapidly in relation to the pressure. The opening pressure is then determined by identifying the point of inflection in the P-V curve where the increase in volume becomes more rapid.
Although this procedure provides an opening pressure which can be used for determining the further treatment of the -patient, it is lacking somewhat insensitivity for determining an optimal opening pressure. For instance, there may be further inflection points in the P-V curve, which inflection points are not immediately identifiable as such. Further it does not guarantee that all collapsed alveoli have been opened properly.